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A crazy, out-of-control person who does things like eat, yell, or dance until everyone else wants to die.
My uncle mad fatty'd the whole family at Thanksgiving. He ate four pies and screamed about the mashed potatoes.
She mad fatty'd the gym by doing 50 push-ups and then crying about her hair.
He mad fatty'd the bus ride by talking to the driver about his ex for 20 minutes.